This is accepted by AXIS-generated client code but in .NET (and other schema-validating tools such as XMLSpy) you'll get an error along the lines of

No type definition found for the type referenced by the attribute 'xsi:type'='ns1:MyResponse' of element <MyResponse>

The error inidicates validation is probably doing the right thing with the xsi:type attribute and checking it - there is actually no type with this name, it's anonymous. Can the behavour in AXIS be changed?

There are a number of ways to get this setting in there. I'd recommend creating a wsdd deploy file for just the globalConfiguration and deploy this to the AXIS AdminServlet as you would do with the deploy.wsdd definitions for the other service(s).
So an example globalConfig-deploy.wsdd: